Deaeration of viscous and plastic materials



1952 L. E. PETERSON ETAL 2,620,894

DEAERATIONJJF VISCOUS AND PLASTIC MATERIALS Filed March 25. 1948 P/EZO ELEC TR/C CRYSTAL INVENTORS. LEROY ERIC PETERSON SIDNEY LEONARD DART Patented Dec. 9, 1952 DEAERATION OF VISCOUS AND PLASTIC MATERIALS Leroy Eric Peterson and Sidney Leonard Dart, Swarthmore, Pa., assignors to American Viscose Corporation, Wilmington, Del., a corporation of Delaware Application March 25, 1948, Serial No. 17,064

Claims.

The present invention relates to a method for removing gas bubbles from viscous liquids. More specifically, the invention is directed to the preparation of viscose for spinning into filaments, films, sheets, and the like, and is particularly concerned with the removal of gas bubbles from viscose preliminary to such spinning or other forming operations. The invention will be described more specifically in connection with the treatment of viscose. During its preparation, viscose is subjected to considerable mechanical agitation by stirring and mixing agitator blades, as well as to the agitation which results from hydraulic turbulence during the transfer of the viscose from one tank to another. This agitation results in the introduction of considerable amounts of gas bubbles into the viscose. Also, the viscose contains naturally developed hydrogen sulfide present therein in the form of small bubbles.

Heretofore, the removal of gas bubbles from viscose has generally been effected by placing the viscose in a tank or the like and evacuating the tank. That procedure has the disadvantage that it requires a plurality of tanks when a continuous spinning or forming procedure is desired, in order that while one of the tanks is under vacuum, at least one other tank is available to supply the viscose to the spinnerets or other forming units, and. yet another is being filled preparatory to evacuation. Special care has to be taken during discharge of the vessel from the degasifying tank that no air is entrained by the vortex developed at the bottom of the tank when the level in the tank becomes low. In order to prevent such inclusion of air bubbles, it has been the common practice to allow a residue or heel to remain in the bottom of the tank, and the heel must be either thrown away or mixed with the next batch introduced into the tank. Various systems have been prepared to overcome the troublesome problems encountered in the removal of gas bubbles from viscose under vacuum.

In accordance with the present invention, highly viscous materials, such as viscose, are divested of gas bubbles under the influence of longitudinal or compressional waves, herein called high frequency sound Waves, that is, sound waves having a frequency of from 1,000 to a billion or more cycles per second, with or without the aid of vacuum or pressure.

.We have found that high frequency sound waves are particularly valuable for use in divesting viscose or like viscous and even plastic .from the viscous material.

materials of gas bubbles. Usually, such materials contain a large number of very small bubbles of low buoyancy. The high frequency sound waves effect coalescence of the small bubbles to large bubbles of greater buoyancy. If the viscous liquid containing minute gas bubbles is placed in a vessel and subjected to the action of high frequency sound waves applied adjacent the bottom of the vessel, the large bubbles formed by coalescence of the smaller bubbles under the action of the waves migrate to the surface of the mass very rapidly due to their increased buoyancy. However, we have found that in degassing such highly viscous liquids as viscose, it is not sufficient merely to effect coalescence of the small bubbles present initially, by the sound waves, in order to successfully remove the gas During the irradiation, a train of sound waves moves continuously upwardly through the mass of viscous material in the field, and each successive wave strikes against the surface of the mass. The turbulence thus created at the surface of the mass results in the development of strong convection currents which tend to return the bubbles to a lower level in the mass before the bubbles have time to break through the viscous material at the surface. If strong convection currents are permitted to develop at the surface, the large gas bubbles formed by the action of the sound waves may migrate to the surface of the mass for practically indefinite periods, and return to the body of the mass without breaking through the viscous material at the surface. In that event, the viscose or the like may b irradiated with the high frequency sound waves for long periods of time without effecting removal of any appreciable amount of gas from the mass.

In accordance with the present invention, viscous and even plastic materials containing small gas bubbles are placed in a vessel and irradiated with high frequency sound waves which effect coalescence of the small bubbles to bubbles of larger size which rise rapidly to the surface of the material, the irradiation being performed under conditions such that the development of convection currents tending to return the bubbles to the lower levels in the mass before they can break through the viscous material, is inhibited.

In its most important aspect, the invention contemplates a method wherein viscose or the like is fed continuously to a vessel supported in a field of high frequency sound waves applied adjacent the bottom of the vessel, downwardly through the sound wave field, and is continuously withdrawn adjacent the bottom of the vessel.

One essential feature of the invention resides in that all portions of the viscose or the like moving downwardly through th vessel, are exposed to the action of the waves. This requirement is met by utilizing a transducer, the transverse dimension of which at least corresponds to the transverse dimension of the vessel. Thus, if the vessel is cylindrical, for example, if it is a pipe having a diameter of 3", a circular transducer, which may be, for instance, a piezoelectric crystal or crystal mosaic having a diameter of at least 3 is used.

Another essential feature of the invention is that, during the irradiation, a head of the viscous material of constant depth must be maintained in the field. We have found that, in order to inhibit or prevent the development of convection currents at the surface of the material being irradiated, the head must have a depth at least twice the transverse dimension of the vessel. For example, if the viscose or the like is irradiated as it is fed downwardly through a pipe having a diameter of 3", the rate of flow of the material is controlled so that a head having a depth of at least 6 is maintained in the field throughout the degasifying operation. So long as the minimum requirement is met, the upper limit for the depth of the head maintained in the field is not critical, and the head may have a depth up to 10 feet. Soon after the irradiation has been initiated, the small bubbles present in the portion of the mass nearest the transducer string together in the form of a chain, and then coalesce to comparatively large bubbles of increased buoyancy which rise upwardly through the field to the surface. The portions of the mass in the immediate vicinity of the transducer are cleared of gas bubbles very rapidly. When the degasification is performed continuously under the conditions described herein, in accordance with which a head having a depth at least twice the transverse dimension of the vessel is maintained in the field, the large bubbles traveling away from the transducer, upwardly through the field and counter-current to the flow of the material moving downwardly in the field, are joined by bubbles entrained in the material entering the field, and a level is reached, away from the transducer, beyond which few, if any, gas bubbles descend to the bottom of the vessel. If the conditions of the invention are observed, the material passing out of the field is free of gas bubbles. It is sent to a suitable collection tank or the like, care being taken to prevent the entrainment of further amounts of air or other gas with it.

A preferred embodiment of the invention is illustrated diagrammatically in the accompanying drawing. In the drawing, there is shown a storage tanlc 2 from which the viscose or the like containing bubbles is withdrawn through conduit 3, provided with valve 4, and delivered to the vessel 5 which may advantageously take the form of a pipe of from 1 to 3 diameter. The vessel is provided with threads which engage the threads on a reducing T 6 supported in a vessel '5 on legs 0. The viscose or the like is withdrawn from the reducing T, which comprises the lower part of vessel 5, through the conduit l0, having valve ll, forwarded to the collection tank (not shown). Tank '5 contains an oil bath in which is positioned a piezoelectric crystal l2 connected by leads 53 to the terminals of the high frequency electric oscillator 10. A diaphragm 15 of resilient material capable of transmitting the sound wave radiations is held between the ring member l6 and a gasket ll. The diaphragm constitutes the bottom of the vessel 5 and the sound waves are transmitted to the viscose moving downwardly in the vessel through the resilient diaphragm.

As will be obvious, the rate of feed of the viscose to the vessel, and the rate of withdrawal thereof from the vessel is controlled by means of the valves i and H, the valves being set so that the head of required depth is maintained at all times in the vessel 5.

The arrangement specifically illustrated may be modified so that the sound waves are applied directly to the viscose or the like, or both directly thereto and indirectly thereto through the bottom wall of the vessel. Although the transducer specifically illustrated is of the piezoelectric crystal type, any transducer capable of converting electrical waves to sound waves having a frequency within the range indicated may be used. If the transducer is of the magnetostrictive or electromagnetic type, the diaphragm which is an integral element of those transducers may itself constitute the bottom wall of the vessel.

The following examples will serve to illustrate the invention:

Example I Using apparatus as shown in the drawing, wherein vessel 5 comprised a pipe of 1 diameter, viscose was passed through a field of sound waves having a frequency of 400,000 cycles per second, at a rate of 5 liters/hour. A head of viscose in the field was maintained substantially constant at a depth of 5".

Example I I Viscose was passed through a pipe as in Example I, supported in a field of sound waves having a frequency of 700,000 cycles per second, at the same rate and under the same conditions as in Example I.

Example III Using apparatus as shown in the drawing, wherein vessel 5 comprised a pipe having a diameter of Sf, viscose was passed through a field of high frequency sound waves having a frequency of 400,000 cycles per second, at the rate of 20 liters/hour. A head having a substantially constant depth of 6 was maintained in the field.

Example IV Viscose was passed through a pipe as in Ex ample I, and irradiated with sound waves having a frequency of 1,000,000 cycles per second. The viscose was fed through the field at a rate of 5 liters/hour, a head of about 3" being maintained in the field.

Example V Viscose was passed through a pipe as in Example I, and irradiated with sound waves having a frequency of 1,500,000 cycles per second. The rate of feed of the viscose through the field was 5 liters/hour. A head 6" deep was maintained in the field at all times.

Example VI Using apparatus as in Example I, viscose was passed through a pipe having a diameter of 3" at the rate of 20 liters/hour, and irradiated with sound waves having a frequency of 700,000

'5 cycles/second. A head 8" deep was maintained in the field.

In all instances, the large gas bubbles formed by coalescence of a multiplicity of the smaller bubbles present initially in the material, under the action of the sound waves, rose to the surface of the material and broke through the viscous material as they reached the surface, so that the gas was released to the atmosphere. The degasific'ation may be performed with the aid of vacuum and/or pressure, but preferably the viscous material is irradiated in a vessel open to the atmosphere, as illustrated herein.

In addition to facilitating removal of gas bubbles from viscous materials, the high frequency sound waves have the effect of continuously homogenizing the material. This homogenization, which proceeds concurrently with removal of the gas bubbles, is of special advantage in the case of viscose. Very commonly, viscose which has been filtered and degassed in accordance with the conventional methods is found to comprise small gels which remain suspended in it. Such gels may be microscopic in size, and are frequently small enough to pass through the filtering devices. They may clog the orifices of the jet, or, if they pass through the jet, they are present in the article formed from the viscose and have a decidedly undesirable effect on the properties, for example, on the dyeing properties of the article. Viscose which is divested of gas bubbles in accordance with the present invention is simultaneously homogenized and any gels remaining suspended therein after the filtration are dissolved or dispersed in the sound wave field. This assures a smoother, more uniform viscose, and final articles, such as filaments or fibers, of improved properties.

Variations and modifications may be made in carrying out the method of the invention without departing from the spirit and scope thereof as defined in the appended claims.

We claim:

1. The method of removing gas bubbles from viscose which comprises continuously feeding a stream of viscose containing gas bubbles downwardly through a vessel supported in a field of high frequency sound waves having a frequency between 400,000 cycles per second and 1,500,000 cycles per second, to thereby subject all portions of the viscose moving downwardly through the vessel simultaneously to the action of the sound waves, continuously withdrawing the viscose from the vessel and correlating the rate of feed of the viscose to the vessel with the rate of withdrawal thereof from the vessel so that a head of viscose having a substantially constant depth at least twice the transverse dimension of the vessel is maintained in the vessel, the sound waves bein applied adjacent "the bottom of the vessel so that the vibrations in the viscose induced thereby originate at the base of the column of viscose comprising the head.

2. The method of claim 1, wherein the sound waves have a frequency of 400,000 cycles per second.

3. The method of claim 1, wherein the sound waves have a frequency of 700,000 cycles per second.

4. The method of claim 1, wherein the sound waves have a frequency of 1,000,000 cycles per second.

5. The method of claim 1, wherein the sound waves have a frequency of 1,500,000 cycles per second.

LEROY ERIC PETERSON. SIDNEY LEONARD DART.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,147,677 Smith Feb. 21, 1939 2,163,650 Weaver June 27, 1939 2,258,630 Smith Oct. 14, 1941 2,363,247 Holder Nov. 21, 1944 2,376,221 Baker May 15, 1945 2,413,102 Ebert et al. Dec. 24, 1946 2,456,706 I-Iorsley Dec. 21, 1948 FOREIGN PATENTS Number Country Date 458,893 Great Britain Dec. 29, 1936 OTHER REFERENCES Ultrasonics, R. W. Boyles, Science Progress, vol. 23, 1928, pages 94-97.

Industrial Applications of Supersonic Vibrations, Freundlick, Transactions, Institute of Chemical Engineers, vol. 15, 1937, page 225. 

1. THE METHOD OF REMOVING GAS BUBBLES FROM VISCOSE WHICH COMPRISES CONTINUOUSLY FEEDING A STREAM OF VISCOSE CONTAINING GAS BUBBLES DOWNWARDLY THROUGH A VESSEL SUPPORTED IN A FIELD OF HIGH FREQUENCY SOUND WAVES HAVING A FREQUENCY BETWEEN 400,000 CYCLES PER SECOND AND 1,500,000 CYCLES PER SECOND, TO THEREBY SUBJECT ALL PORTIONS OF THE VISCOSE MOVING DOWNWARDLY THROUGH THE VESSEL SIMULTANEOUSLY TO THE ACTION OF THE SOUND WAVES, CONTINUOUSLY WITHDRAWING THE VISCOSE FROM THE VESSEL AND CORRELATING THE RATE FEED OF THE VISCOSE TO THE VESSEL SO THAT A HEAD OF DRAWAL THEREOF FROM THE VESSEL SO THAT A HEAD OF VISCOSE HAVING A SUBSTANTIALLY CONSTANT DEPTH AT LEAST TWOCE THE TRANVERSE DIMENSION OF THE VESSEL IS MAINTAINED IN THE VESSEL, THE SOUND WAVES BEING APPLIED ADJACENT THE BOTTOM OF THE VESSEL SO THAT THE VIBRATIONS IN THE VISOSE INDUCED THEREBY ORIGINATE AT THE BASE OF THE COLUMN OF VISCOSE COMPRISING THE HEAD. 